Various abbreviations that appear in the following description and in the Figures are defined as follows:
3GPPthird generation partnership projectCAZACconstant amplitude zero autocorrelationCPcyclic prefixDFTdiscrete Fourier transformeNBE-UTRAN Node-B, evolved Node-BE-UTRANevolved universal terrestrial radio access networkIDFTinverse DFTLTElong term evolution of UTRAN (E-UTRAN)Node-B base stationOFDMAorthogonal frequency division multiple accessRACHrandom access channelUEuser equipment, such as a mobile station or mobile terminalULuplink (UE to Node-B)UTRANuniversal terrestrial radio access networkZCZadoff-Chu
In E-UTRAN standardization of 3GPP there have been extensive discussions related to the non-synchronized RACH preamble structure. The Zadoff-Chu CAZAC sequence has been agreed upon as a preamble sequence for LTE UL. Its ideal periodic autocorrelation properties have been seen as beneficial for use as the RACH preamble, for example, multiple preambles are obtained from a single base Zadoff-Chu sequence with cyclic shifts of the sequence. Zadoff-Chu sequences of odd length are given by:
                    a        u            ⁡              (        k        )              =          exp      ⁡              (                              -            j                    ⁢                                          ⁢          2          ⁢                                          ⁢          π          ⁢                                          ⁢          u          ⁢                                    qk              +                                                (                                                            k                      2                                        +                    k                                    )                                /                2                                                    N              G                                      )              ,where q is an integer and the sequence index u defines the base sequence. In the following,au,d(k)=au(k−d mod NG)refers to the d th cyclic shift of sequence au. These sequences were under previous consideration for E-UTRAN.
The multiple access scheme of the LTE UL is single carrier frequency division multiple access (SC-FDMA) combined with time division resource allocations (TDMA). A part of the frequency and time resources are reserved for transmission of random access preambles. The current working assumptions in 3GPP are that the RACH preamble is transmitted on a 1.08 MHz bandwidth and 64 preambles are used in each cell. A preamble, preceded by a cyclic prefix, consists of a single 0.8 ms Zadoff-Chu sequence, repeated in burst formats 2 and 3 of frame structure type 1, and followed by a guard period. Instead of a single 0.8 ms Zadoff-Chu sequence, a repetition of a 0.4 ms Zadoff-Chu sequence (thus totaling 0.8 ms) was also considered. In the current working assumptions, the cyclic shifts available for the preamble are restricted in environments with high velocity terminals. In the following, the length of the Zadoff-Chu sequence is designated with Ts.
Reference may be had to R1-070377, 3GPP TSG RAN WG1 #47bis, Sorrento, Italy, Jan. 15-19, 2007, “Restricted sets of RACH preamble signatures for environments with high Doppler shifts”, Nokia.
Alternative ways of generating the single carrier preamble signal are shown in FIG. 6. The Scheme A is for time domain processing and the Schemes B and C for frequency domain processing. LTE is likely to adopt scheme B. The processing in Scheme B is in accordance with the DFT-S-OFDMA system used for transmission on the scheduled resources, while Scheme C simplifies processing compared with Scheme B. The filtering blocks in B and C may not be present in the final system.
The properties of preamble detections due to frequency offset can be described as follows.
The NG cyclic shifts of a Zadoff-Chu sequence can be seen as an orthogonal base of NG-dimensional space.
A frequency offset of 1/Ts, where Ts is the duration of the preamble, rotates the transmitted sequence from the original direction to the direction of another cyclic shift B. As a result, the received sequence is orthogonal with the transmitted one. The cyclic shift B depends on the sign of frequency offset as well as on the u-index of the sequence.
When the frequency offset is less than 1/Ts, the rotation is not restricted to the plane defined by the original sequence and the cyclic shift B. However, the largest components are in these directions.
The cyclic shifts of au,d(k) that correspond to the +/−1/Ts frequency offsets are au,(d+coff mod NG)(k) and au,(d−coff mod NG)(k), respectively. When the preamble is defined in the time domain (Schemes A and B in FIG. 6), the cyclic shift offset is given by coff=(NGm−1)/u, where m is the smallest positive integer for which coff is integer. On the other hand, if the preamble is defined in the frequency domain, i.e., transmission is according to the Scheme C in FIG. 6, the cyclic shift offset coff=u. In the case of Scheme C of FIG. 6, au(k)=IDFT(Au(n)), where IDFT( ) is inverse discrete Fourier transformation, and
            A      u        ⁡          (      n      )        =            exp      ⁡              (                              -            j2π                    ⁢                                          ⁢          u          ⁢                                    qn              +                                                (                                                            n                      2                                        +                    n                                    )                                /                2                                                    N              G                                      )              .  
In the following denote {au,(d−coff mod NG)(k), au,d(k) au,(d+coff mod NG)(k)} a cyclic shift triplet of au,d(k), and refer to {au,(d−coff mod NG)(k), au,(d+coff mod NG)(k)} as a frequency cyclic shift pair of au,d(k), since they are obtained from au,d(k) by performing a cyclic shift of +−1 in the frequency domain. A frequency cyclic shift means that a cyclic shift is done to the frequency domain presentation of a sequence.
In other words, a frequency cyclic shift of +/−1 for au,d(k) is the cyclic shift of the root sequence au(k), that is equal up to a phase rotation with the sequence obtained when the phase of elements of sequence au,d(k) are rotated corresponding to a frequency offset of +/−1/Ts, correspondingly. A frequency cyclic shift of +/−1 may also be given by a cyclic shift offset coff. au,(d+coff mod NG)(k) and au,(d−coff mod NG)(k), correspondingly
When detecting a RACH preamble au,csn(k), that is, the csnth cyclic shift of a Zadoff-Chu sequence au(k), the cyclic shifts au,csn(k), . . . , au,csn+D−1(k) corresponding to a timing uncertainty window are examined. The width of timing uncertainty window D is related to the maximum expected round trip propagation delay and channel delay spread. This set of au,csn(k), . . . , au,csn+D−1(k) cyclic shifts is referred to as the timing uncertainty window of the preamble au,csn(k) in the following and is distinct for each preamble.
Different RACH preamble detection methods are considered in R1-063214, 3GPP TSG RAN WG1 #47, Riga, Latvia, Nov. 6-10, 2006, entitled: Non Synchronous Random Access Design for High Doppler Conditions”, Texas Instruments.
Reference can also be made to R1-070226, 3GPP TSG RAN WG1 #47bis, Sorrento, Italy, Jan. 15-19, 2007, “Non-synchronized Random Access Design under Frequency Offset”, LG Electronics, and to R1-070227, 3GPP TSG RAN WG1 #47bis, Sorrento, Italy, Jan. 15-19, 2007, “Ways to Mitigate Frequency Offset with CAZAC Cyclic Shift”, LG Electronics.